Terpene alcohols and esters occur naturally in many essential oils. The oils, or their components, are ubiquitous in flavor and fragrance applications. Synthetic routes to the more important terpene esters, such as linalyl acetate, geranyl acetate, and citronellyl acetate, have been developed and commercialized. Linalyl acetate occurs naturally in lavender (>50%), bergamot (27%), petitgrain bigarade (44%), clary sage (45-60%), and neroli (6%) oils. Geranyl acetate is found in coriander (16%), palmarosa (8-17%), and citronella (5-8%) oils, among others. Terpinyl acetate is a major component (24%) of cardamom oil. Other naturally occuring terpene esters include citronellyl, lavandulyl, bornyl, and neryl acetates.
As with other alcohols, terpene alcohols are conveniently converted to the corresponding acetate esters by reacting them with acetic anhydride. The reaction is normally performed in the presence of a catalyst. Reported catalysts are common, such as p-toluenesulfonic acid or sodium acetate, or more esoteric, such as copper(II) triflate (Tetrahedron Lett. 40 (1999) 2611), bismuth(III) triflate (J. Org. Chem. 66 (2001) 8926), copper(II) tetrafluoroborate (Synthesis (2004) 111), and polymer-bound amines (J. Org. Chem. 50 (1985) 3928; J. Appl. Polym. Sci. 84 (2002) 1067).
More commonly, however, the acylation is performed in the presence of a stoichiometric amount of a low-boiling amine such as pyridine or triethylamine (see U.S. Pat. No. 4,008,256 and Angew. Chem. I.E. (1978) 569), which accelerates the acylation and neutralizes the acetic acid by-product. A catalytic amount of 4-(dimethylamino)pyridine or 4-(1-pyrrolidino)pyridine can be used to further speed the reaction, particularly for difficult acylations such as ones that involve tertiary terpene alcohols. The pyridinium acetate or triethylammonium acetate salt is eliminated by water washing using a typical organic workup. Such common workups are illustrated by the preparation of linalyl acetate from linalool (Synthesis (1972) 619 and Angew. Chem. I.E. (1978) 569 at page 573) and, more recently, by the preparation of 1-cyclohexyl-1-methylethyl acetate from 2-cyclohexyl-2-propanol (U.S. Pat. No. 7,064,102).
Unfortunately, it is often impractical, especially in an industrial context, to generate a waste stream that contains pyridinium or trialkylammonium acetate salts. Ideally, the acetic acid generated during acylation would be recovered. Thus, a preferred process would acylate terpene alcohols without using a molar equivalent of pyridine or triethylamine, and without generating a waste stream.
In the typical acylation process with acetic anhydride and a low-boiling amine, the amine helps to avoid acid-catalyzed side reactions (such as dehydration or isomerization) by neutralizing acetic acid as it forms. Such side reactions are particularly important when the terpene alcohol is tertiary and/or unsaturated (as with linalool). Consequently, simply eliminating the low-boiling amine is not a workable solution; the side reactions still need to be avoided.
Moreover, fragrance-quality terpene esters must be substantially free of nitrogen-containing impurities. Thus, it is unacceptable to use an excess of pyridine or triethylamine because traces of these amines, if used for acylation, will usually be present in even a carefully distilled terpene ester.
The flavor and fragrance industry would benefit from an improved way to make and purify terpene esters. In particular, a way to acylate terpene alcohols while recovering acetic acid and avoiding aqueous waste streams is needed. A valuable process would be simple to practice without an esoteric catalyst and would afford fragrance-quality terpene esters. The ability to recover and reuse the catalyst is also desirable. Ideally, the process could be used to make a variety of terpene esters known to be valuable flavor or fragrance components.